Address issues brought up during review
Address issues brought up during review

- Avoid depending on F[_] having an implementation that catches exceptions
- Do not assume that a Step with an Emit(Nil) is done
- Return a successful F[_] of None when the process is killed as opposed to a failed F[_]

zaneli/fs2,drostron/scalaz-stream,fthomas/scalaz-stream,danbills/scalaz-stream,lowtalker/scalaz-stream,shawjef3/scalaz-stream,FranklinChen/scalaz-stream,kanterov/scalaz-stream,djspiewak/scalaz-stream,arzig/scalaz-stream,jaapterwoerds/scalaz-stream,japgolly/scalaz-stream,lukiano/scalaz-stream,sellout/scalaz-stream